There's a vast difference between the feel of an MMO and a hack and slash dungeon crawler like Diablo. No matter how crappy an MMO is, it generally feels like a living world with different environments. Games like Diablo don't have that feeling. Even playing with other people, it's still a dungeon crawler. You've just got another person or two hitting the loot pinata.

Don't get me wrong, I love the mindless wholesale slaughter at times. I've spent an embarrassing amount of time on games like Torchlight and TitanQuest as well as Diablos past and present, both solo and co-op. I used to live and breathe singleplayer and co-op games until I was introduced to MMOs by way of Asheron's Call in 2000. After that, it was all MMOs all the time. Games like this fill in the times where I'm not multiboxing and don't have anything I want to do in an MMO.

Multiboxing = turning an MMO into a singleplayer? Maybe if all you do is sit in AV and faceroll people all day. But in general, MMOs give far more freedom of what to do than games like Diablo.